The enzyme thymidylate kinase phosphorylates the substrate thymidine 5'-phosphate (dTMP) to form thymidine 5'-diphosphate (dTDP), which is further phosphorylated to dTTP for incorporation into DNA. Ehrlichia chaffeensis is the etiologic agent of human monocytotropic erlichiosis (HME), a potentially life-threatening tick-borne infection. HME is endemic in the United States from the southern states up to the eastern seaboard. HME is transmitted to humans via the lone star tick Amblyomma americanum. Here, the 2.15 Å resolution crystal structure of thymidylate kinase from E. chaffeensis in the apo form is presented.